Lattice frames comprise elongate members, usually tubes, that lie on intersecting longitudinal axes and may be joined by node structures where those axes intersect. Such node structures may be cast in one piece or may be fabricated from parts by welding.
An example of a lattice frame is a ‘jacket’ substructure for a wind turbine or other offshore structure, which will be used to exemplify the invention in the description that follows.
Jackets have been used in the oil and gas industry for many years; they have recently also found favour in offshore wind turbine applications where water depth in excess of about twenty meters makes it impractical to use a traditional monopile or gravity-based substructure.
A jacket for an offshore wind turbine is shown in FIG. 1. The jacket 10 shown in FIG. 1 is of conventional overall shape that reflects the prior art but includes various node structures in accordance with the invention, which will be described in detail later with particular reference to FIGS. 13 to 16.
In the example shown in FIG. 1, the jacket 10 comprises four upwardly-converging tubular legs 12 that collectively define a truncated four-sided pyramid of square horizontal cross-section. A three-sided pyramid of tripod configuration is also possible, in that case having a triangular horizontal cross-section.
Each face of the pyramidal jacket 10 comprises a series of cruciform X-braces 14 of downwardly-increasing size, disposed between upper and lower horizontal struts 16 that extend between adjacent pairs of legs 12. Each X-brace 14 comprises four tubular diagonal struts 18 that converge inwardly to connect at a central X-node structure 20 and diverge outwardly toward respective K-node structures 22 that connect the diagonal struts 18 to the legs 12. Each K-node structure 22 in this example is a double-K that connects four diagonal struts 18 to the associated leg 12.
The uppermost and lowermost nodes on each leg 12 may be regarded as Y-node structures 24 as they connect only one diagonal strut 18 of each face of the jacket 10 to the associated leg 12, although each Y-node structure 24 in this example is a double-Y that connects two diagonal struts 18 of adjacent faces to the associated leg 12. However, the Y-node structures are also akin to K-node structures or double-K node structures as they also connect the horizontal struts 16 to the legs 12.
The legs 12 of the jacket 10 will seat into a pre-piled foundation structure (not shown) that is fixed to the seabed, in water whose depth is potentially in excess of thirty meters. The jacket 10 is tall enough to protrude above the surface so that a wind turbine tower may be mounted on top, clear of the water. In practice, the jacket 10 will be surmounted by a transition piece for supporting a typically tubular wind turbine tower and providing a working platform around its base, but this transition piece has been omitted from FIG. 1.
It is important for a node structure of a lattice frame to minimise stress concentrations. It is also desirable for a node structure to be compact for low material cost and to minimise resistance to water movement due to waves and tides, which imparts lateral loads to the jacket structure.
If the node structure is to be fabricated, it is desirable for that node structure to be easy to fabricate in various locations around the world with minimal tooling cost. However, whilst preferred aspects of the invention relate to fabricated node structures, other aspects relate to shape features that are independent of the means of manufacture and so may be embodied in a non-fabricated node such as a cast node.